I saw this endgame tactic in a game between Honinbo Shuei and Yasui Sanei

(;C[Please play the best endgame in this position.]AB[qp]AB[qn]AB[pn]AB[on]AW[rm]AW[qm]AW[pm]AW[ol]AP[goproblems]
(;B[rn];W[sn];B[so];W[sm];B[ro]C[This result is not good enough. There is a better one.
])
(;B[sn];W[rn];B[ro]LB[sm:A]TR[qp]C[If white takes sente now, black can play 'A' in sente. This is two points better for black. If white plays 'A', black can ignore thanks to the marked stone. :-)RIGHT]))